Use the video out port and borrow an external monitor to see what is going on. You can then rescue whatever you want off the mashed machine over a network, USB or optical disk. You may need to toggle the output to an external screen by use of function keys or key combinations. For the laptop in question I think it's ctrl + F7 Or You don't need a third drive, you can keep an install of OS X on a second drive and use it as a Time Machine backup for the first drive, then boot from the second and restore from Time Machine on it. I've done this just to see if it was possible.